Analysis On Sedimentary Age, Provenance And Sedimentary Environment Of Sunjiagou Formation In Yiyang Area, Western Henan Province

During the transition period from Permian to Triassic,the largest extinction event occurred in Phanerozoic in the world,and great changes have taken place in paleostructure,Paleontology and paleoclimate.Therefor,the mass extinction and global change between Permian and Triassic are important events in the evolution of the earth.The Sunjiagou Formation is a sedimentary record of the Permian-Triassic transition period of the North China Block.The study of its sedimentary environment and its controlling factors is of great significance to the analysis of the great changes in the Paleoenvironment of the North China Block during this transition period.The Sunjiagou Formation in Yiyang area,western Henan,in the southern part of North China Block,is mainly a continental sedimentary stratum in the early Triassic,which is well exposed,and is an important material for studying the continental sedimentary environment during the transition period from Permian to Triassic.In this paper,the sedimentary environment and its controlling factors of the Lower Triassic Sunjiagou Formation in Yiyang area,western Henan Province are analyzed by means of petrology,mineralogy,sedimentology and geochemistry.And the following conclusions are drawn:(1)The lithology of the lower member of Sunjiagou Formation in Yiyang area of Western Henan Province is mainly fine sandstone and siltstone,while the rhythmic layer of the upper member is mainly composed of carbonate rocks and mudstone.According to petrological characteristics,sedimentary sequence and sedimentary structure,delta to lacustrine facies deposits are developed from lower to upper part,which is a sedimentary environment in which water body gradually becomes shallow.(2)According to the maximum sedimentary age(YSG)determined by detrital zircon,the maximum sedimentary age(YSG)of yellowish medium-fine feldspar quartz sandstone in the bottom of lower member of Sunjiagou Formation in Yiyang area,western Henan Province is 248±6.9Ma,and that of purple-red mudstone in the bottom of upper member of Sunjiagou Formation is 247±6.7Ma.Therefore,the sedimentary age of Sunjiagou Formation belongs to the Early Triassic;According to the provenance analysis determined by detrital zircon,the main provenance of the lower member of Sunjiagou Formation comes from the northern Inner Mongolia uplift and the metamorphic basement of the North China Block,the secondary provenance comes from the North Qinling orogenic belt,while the main provenance in the upper member turns to the North Qinling orogenic belt.(3)Based on the thin section,X-ray diffraction and carbon and oxygen isotope analysis of thin carbonate rocks in the upper member of Sunjiagou Formation,the rhythmic layer composed of thin carbonate strata and overlying purple mudstone strata in the upper member of Sunjiagou Formation is generally related to the lake basin subsidence,but its formation process is mainly related to climate.Carbonate beds are the result of sudden increase in rainfall,strong hydrolysis of mudstones deposited,and massive loss of calcareous matter and convergence into Lake basins.And the original rocks of purple mudstone overlying carbonate strata are caused by the shallowing of the lake basin.The variation of carbon and oxygen isotopes of carbonate rockss in the upper member of Sunjiagou Formation from the lower part to the upper part reveals that the lake environment at that time underwent evolution from relatively open to relatively closed and then to relatively open,and the generation of lake environment and the evolution of lake environment in the upper member of Sunjiagou Formation may be related to the uplift of the North Qinling orogenic belt.(4)According to the variation of element content of mudstone in rhythmic beds composed of carbonate rocks and mudstones in the upper part of upper member of Sunjiagou Formation in Yiyang area,western Henan Province,it is considered that they are the result of chemical weathering when water body becomes shallow and exposes to the surface.When rainfall suddenly increases,the original rocks of these purple mudstones will undergo intense hydrolysis,accompanied by massive loss of calcium and enrichment of iron,and loss of calcium is concentrated in the lake basin with the increase of lake level due to the large rainfall.Then,the purple mudstone protolith has been deposited on the upper part of the carbonate rock beds,showing a gradually shallow sedimentary sequence due to the shallow water body and the increase of provenance supply.The change of element content in mudstone is the result of chemical weathering.